Known electric power utilities or transmission system operators own and operate electric power transmission networks interconnecting sites, such as power sources and substations, which despite being distant from each other some 100 km or more, have to be coordinated in one way or the other. Across their utility communication systems, a variety of messages are exchanged over long distance communication links between distant sites of the utility in order to safely transmit and distribute electric energy. For some of these messages, and considering a specific implementation for teleprotection commands, the transmission delay between transmitter and receiver can be detrimental to the system and should not exceed a few milliseconds up to some 10 ms. For any time-sensitive application, increased transmission delay, or packet loss, can result in malfunction or even damage of a power system.
Teleprotection commands for known distance protection schemes in electrical power networks can be transmitted by remote tripping devices or protection signal transmission devices, also known as teleprotection devices, over dedicated point-to-point communication links. Physical signal links can involve radio waves or fiber optics, and in an exemplary embodiment, the protection signals can be transmitted over pilot wires, analog leased lines, voice channels of analog or digital communication systems, or even high-voltage electricity transmission lines, the latter being known as power line communication (PLC). Dedicated teleprotection systems permanently monitor the state of a communication system, for example, by means of dedicated loop test messages periodically exchanged.
For securely transmitting messages over long distances from one site to the other, the utility can also rely on a Wide-Area communication Network (WAN). In the present context, a WAN can be a dedicated point-to-point communication link between two sites based on e.g. optical fiber or pilot wires, a connection-oriented communication network with a guaranteed data rate such as Ethernet over SDH/HDLC, or a packet-oriented communication network interconnecting a number of sites of the utility, and comprising a plurality of specific network elements such as switches, repeaters and possibly optical transmission media at the physical layer.
WO 2010/081798 is directed to increased reliability of communication over a non-deterministic communication channel, and is suited for inter-substation teleprotection in electric power systems. A communication channel is being monitored based on regular network traffic, e.g., by evaluating data traffic, messages or data packets carrying real-time operational data as a payload. A permanent determination of a channel quality, including generating an alarm in case the channel quality is found insufficient, is based on an evaluation, at a receiving node, of data packets continually, or repeatedly, transmitted by a sending node. In packet oriented WANs, the repeatedly transmitted data packets can be considered as replacing a known guard signal in analogue teleprotection systems or known guard frames in digital teleprotection systems.
A communication standard for communication between the devices of a Substation Automation system has been introduced as part of the standard IEC 61850 entitled “communication networks and systems in substations”. For the exchange of non-time sensitive messages within substations, IEC 61850-8-1 specifies the Manufacturing Message Specification (MMS, ISO/IEC 9506) protocol based on a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet as physical medium. For the exchange of time-sensitive event-based messages within substations, IEC 61850-8-1 specifies the Generic Object Oriented Substation Events (GOOSE) and the Generic Substation State Events (GSSE) directly on the Ethernet link layer (OSI-Layer 2) of the communication stack. Hence, the standard defines a format to publish event-based messages as multicast messages on an industrial Ethernet. Notably, as in known IT communication networks with quasi-permanent traffic, the routers and switches of a substation do not provide for an intra-substation communication network supervision mechanism.
The IEC 61850 standard is concerned with communication systems and communication networks or Local Area Networks (LANs) within substations. Modelling of applications involving communication between substations such as teleprotection is thus outside the scope of the present version of the standard. In order to extend the concepts of the standard to the communication between substations, chapter 8 entitled “Communication Aspects” of the report IEC/TR 61850-90-1 (E) entitled “Communication networks and systems for power utility automation—Part 90-1: Use of IEC 61850 for the communication between substations” proposes two approaches.
FIG. 1 schematically shows two known inter-substation communication alternatives in accordance with known implementations.
As shown in FIG. 1, the known “tunneling” approach (FIG. 1, top), provides that messages are forwarded transparently, e.g., without any loss of information through message conversion at the interface between a LAN of a substation and the WAN connecting the substations. No loss of information implies that every bit of the original message is transmitted, which precludes e.g., stripping of Layer-2 MAC address by a router at the interface. This approach presumes the availability of sufficient bandwidth for transmission of an entire, uncompressed message according to IEC 61850 within a maximum time delay of the order of 10 ms. The tunneling approach models only the command transfer function of the teleprotection communication, but ignores the channel supervision and monitoring function that nevertheless form an important part of a known teleprotection communication.
The above-mentioned report IEC/TR 61850-90-1 (E) does not address any detail relating to implementation of the tunneling approach. As GOOSE/GSSE frames are multicast on OSI Layer-2, their transmission over Layer-3 or IP networks between substations is not entirely straightforward. Exemplary ways to implement tunneling in packet-oriented networks can comprise i) assigning the different substations to the same Layer-3 subnet and using Virtual Local Area communication Network VLAN identifiers to separate the intra-substation traffic from the inter-substation traffic, and to prevent the GOOSE/GSSE frames from flooding the subnet, ii) manual programming of the switches at the LAN/WAN boundary such that native Layer-2 GOOSE/GSSE frames with specific MAC source addresses, Ethertype protocol definition and/or VLAN-ID are forwarded to the WAN channel, or iii) using adapters or specially adapted switches for tunneling the unmodified GOOSE/GSSE frames encapsulated in Layer-3 packets over Layer-3 networks (e.g. Layer-2 tunneling protocol).
FIG. 1 also shows the known “gateway” approach (FIG. 1, bottom). The “gateway” approach provides that messages to be exchanged between the substations are modified for transmission over the WAN, in other words, the relevant information content is extracted from the intra-substation messages according to IEC 61850 and embedded in a different inter-substation message. Thus, from an IEC 61850 point-of-view, the message transmission is not transparent. For teleprotection applications, this approach allows using known teleprotection equipment, which incorporates channel supervision and monitoring functionality and which is designed to reliably and securely convey protection commands with low delay over communication channels with limited bandwidth such as radio links, copper wires or power line carriers. To this purpose, on the sending side of the WAN link, the commands should be extracted from the IEC 61850 messages carrying these commands, while on the receiving side, the original IEC 61850 messages should be reconstructed using the commands delivered by the teleprotection equipment and based on additional information configured or transmitted earlier and independently of the command.